1. Field of the Invention
This invention relates to an automatic adaptive equalizer and more specifically to such an equalizer featuring an improved reset function by which resynchronization can be achieved in a shorter time duration as compared with a known equalizer. The present invention is most suited for use in a digital radio transmission system.
2. Description of the Prior Art
A digital radio transmission system is susceptible to multipath fading or the like and invites waveform distortion of the transmitted signal, which degrades signal quality and which may cause a short break in transmission reception. In order to minimize these problems, it is the current practice to employ an automatic adaptive equalizer using a transversal filter or a decision feedback loop.
The adaptive equalizer in a digital radio transmission system, however, has encountered the difficulty that distortion of the transmitted signal is apt to exceed the capability thereof. More specifically, upon the distortion reaching a level at which the equalizer is unable to deal with same, asynchronism of clock and carrier signals in a demodulator is induced and results in asynchronism of the control loop of the transversal filter. These conditions induce signal distortion within the equalizer itself. This means that even if the distortion of the transmitted signal falls within the capability of the equalizer, the synchronized state is not automatically restored in the equalizer. A known approach to overcoming this problem is to reset the tap gain controllers of the transversal filter to their initial states upon the occurrence of asynchronism in the demodulator. This prior art technique maintains the adaptive equalizer at reset until resynchronism of the clock and carrier in the demodulator occurs. Accordingly, as the equalizer remains inoperative during this time period, a drawback is encountered in that the control loop of the equalizer is not brought back into synchronization unless the waveform distortion of the transmitted signal is lowered to a considerable extent.